Enable the Custom layout first option (so that sym+X works without opening the symbol keyboard).

Press Customize layout.

Add the characters you want to the on-screen layout. Lowercase characters like ä, ö can be found on the Accents tab, uppercase characters have to be added via the Custom tab using Unicode codepoints (e.g. Ä = U+00C4, Ö = U+00D6).

The configured characters can now be input using sym+X on the physical keyboard.

BlackBerry Priv

Note that BlackBerry phones reset the keyboard layout every time the phone is restarted. There is an option in the app to provide a boot shortcut to layout settings.